1. Field of the Invention
The invention relates in general to a well control method and apparatus and in particular relates to the control of a well drilled from a floating drilling vessel while establishing conductor casing in the well. The floating drilling vessels include not only drilling barges, but also ship-like vessels that look like a normal vessel with a drilling rig on it, and semi-submersible vessels, also called column stabilized units, or "semis". Still more particularly, the invention relates to controlling kicks in a well after the marine riser has been removed and during operations where conductor casing is lowered into the hole and cemented to the structural casing and the open hole therebelow.
2. Description of the Prior Art
In drilling operations from a floating vessel, the well is started or spudded by using a heavy steel template to guide the bit to the right spot on the ocean floor. The template has four attached guidelines to guide the equipment to the well before the riser has been run. The template is run to the seabed on drill pipe and released mechanically.
A thirty-six (36) inch bit is run and guided by arms riding on the guidelines so that the bit enters the hole in the template. The guide arms may be of the breakaway variety that are released by the parting of shear pins or the like after the bit has entered the hole in the template. The arms are retrieved to the surface by means of attached tugger lines.
The thirty-six (36) inch hole is drilled from eighty to three hundred feet below the mud line. The drilling tools are then removed and thirty (30) inch casing called "structural casing" is run into the hole and cemented in the hole.
The depth of the thirty-six (36) inch hole is determined taking into account the ability of the sea floor surface to support the wellhead and equipment with the vertical loading or overturning moment used as the criteria for design. If a riser is to be used while drilling the hole for the second casing, called "conductor casing", the ability of the formation to withstand the hydrostatic pressure of the drilling fluid, called "mud", in the riser must be considered also. Typically the structural casing string extends to one hundred (100) feet beneath the seabed while the conductor casing string typically extends to one thousand (1000) feet and more below the sea floor.
Attached to the top of the thirty (30) inch structural casing is a thirty (30) inch wellhead housing with a permanent guide base attached thereto. This housing and guide base is lowered onto the ocean floor until the permanent guide base rests upon the drilling template.
The guide base has guide posts for supporting a blowout preventer stack. The posts are usually hollow and the guidelines are inserted into the hollow guide posts before the permanent guide base is lowered onto the ocean floor for resting on the template.
The prior art methods for drilling subsea wells from floating vessels have typically included as the next step the connection of a marine drilling riser to the thirty (30) inch wellhead housing. A hydraulic connector secures the riser to the wellhead housing. Typically the prior art methods have drilled through the marine riser and through the structural casing into the subsea formations with a pilot hole to the depth to which the conductor casing is desired to be placed. The conductor casing is used to seal off very low integrity formations. It has typically been set to about one thousand (1000) feet or more below the mud line.
In many areas of the world, shallow gas formations have caused blowouts, and consequently, the drilling industry typically uses a diverter system attached to the top of the riser while drilling the hole for the conductor casing. Flow diverters are low pressure annular preventers used to direct the flow of drilling fluids away from the rig floor. In addition to the usual hazards of fire and explosion, a gas blowout at sea can sink a vessel. Gas in the water lowers the density of the fluids supporting the vessel, sometimes to the point where the vessel loses its reserve buoyancy and may even sink. Thus prior art drilling operations have developed the approach of bringing pressurized gas or fluid in the drilling mud to the surface via the riser and venting via a vent line--diverter system while re-establishing control of the well, rather than venting the well subsea.
In deep water drilling, a dump valve or a lift line has been used to decrease the hydrostatic pressure at the wellhead. Dump valves, capable of being opened to dump the cuttings to the seabed, have been located near the bottom of the riser. A lift line from the vessel to the bottom of the riser has been provided through which water, mud or inert gas has been injected to decrease the hydrostatic pressure.
With the riser in place, a pilot hole is drilled through the riser. The pilot hole is then opened to twenty-six (26) inches by means of a collapsible reamer. Before the riser is pulled, the mud is conditioned and a weighted mud is applied to the well to account for the loss in hydrostatic pressure caused by pulling the riser. The next step in the drilling process calls for inserting twenty inch conductor casing with an eighteen and three quarter (183/4) inch wellhead housing attached to the top thereto into the hole. Prior art drilling methods have required that the riser be pulled because its inside diameter is too small to accept the twenty (20) inch conductor casing with the eighteen and three quarter (183/4) inch wellhead housing attached.
Having pulled the riser, the eighteen and three quarter (183/4) inch wellhead housing and conductor casing have typically been run and cemented with the return to the sea floor. Although most operators use twenty inch conductor casing with an eighteen and three quarter inch wellhead housing, a few well operators may use a conductor string of eighteen and five eighths (185/8) or twenty (20) inch casing topped by a sixteen and three quarter (163/4) inch wellhead housing. The wellhead housing, typically one of eighteen and three quarter (183/4) inch size, is landed by means of complementary landing shoulders within the thirty (30) inch wellhead housing attached to the structural casing. After the riser has been pulled and while the conductor casing is being run and cemented into the well, the well has been without effective control.
Thus, the prior art drilling procedures have called for removing the riser and then running the twenty (20) inch conductor casing with no protection against kicks, while lowering it to and within the thirty (30) inch structural casing and finally into position; during this time, the hole is open to the sea. The wellhead housing attached to the top of the conductor casing is later used to connect the primary blowout preventer stack that will be used for the rest of the drilling program. The conductor casing is then cemented into position all the way back to the ocean floor.
During the time that the marine riser is removed and the conductor casing is being prepared to run and is run into the thirty (30) inch structural casing hole, the hole may have experienced a blowout and, in blowing out, may have caved in or cratered. The drilling operator may find when the conductor casing is being lowered for insertion into the thirty (30) inch structural casing hole that there is a heavy flow of gas out of the hole and that it is impossible to stab the conductor casing into it. Thus, there is no control of the well. The only procedure left open to the operator is to wait until the well quits flowing. Such waiting may take hours to days.
If the operator finds that the well is out of control, he may elect to return the conductor casing back to the surface and to run drill pipe down to the hole and try to stab it into the hole. If the operator is successful in this procedure and the hole has caved in, the well may have to be redrilled.
Another procedure available to the operator if the blowout is substantial, may involve pumping quick set cement into the hole and abandoning the well. So there has existed a problem with prior art drilling from floating drilling rigs of there being a time in the drilling procedure in which the hole is without effective control. Even worse, the operator has had no means to know the pressure conditions of the drill hole once the marine riser is removed, and indeed may not fully realize the condition of the hole until he goes back down with equipment seeking to lower the conductor casing into the hole.
An object of this invention is to provide a method and apparatus for controlling a subsea well after the marine riser has been removed and during the time that twenty (20) inch conductor casing is being lowered and cemented into the well.
It is a further object of the invention to provide a means for monitoring the well pressure in the hole after the marine riser has been removed.
It is a further object of the invention to provide casing guidance and stripping apparatus whereby the conductor casing may be lowered into the well while maintaining control over the well at all times.